Pre-sintered blank for dental purposes

ABSTRACT

Pre-sintered blanks based on lithium disilicate glass ceramic are described which are suitable in particular for the preparation of dental restorations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application of International patentapplication PCT/EP2013/059700 filed on May 10, 2013, which claimspriority to European patent application No. 12167760.3 filed on May 11,2012, the disclosures of which are incorporated herein by reference intheir entirety.

The invention relates to a pre-sintered blank for dental purposes basedon lithium disilicate glass ceramic which blank is suitable inparticular for the preparation of dental restorations.

Reports on the use of pre-sintered blanks in dentistry have already beenmade in the state of the art.

WO 2010/010087 describes porous silicate-ceramic shaped bodies which areprocessed to form veneers for dentistry. The shaped bodies are to have aparticular density in order to prevent damage during the machining withmilling or grinding systems, e.g. due to the material bursting, and tobe suitable for the selected system.

U.S. Pat. No. 5,106,303 describes the preparation of tooth crowns andinlays by copy milling of compacted ceramic bodies which can optionallybe pre-sintered. To achieve the desired geometry, the bodies are milledto an enlarged shape in order to take into consideration the shrinkagethat occurs during the subsequent sintering to the desired high density.Aluminium oxide, which can optionally include strengthening additives,is used in particular as ceramic material.

U.S. Pat. No. 5,775,912 describes pre-sintered dental porcelain pellets,from which a tooth structure is milled by means of CAD/CAM systems. Thistooth structure is embedded in embedding material, sintered and removedfrom the embedding material in order to produce the desired dentalrestoration. The dental porcelains used are glass ceramics based onleucite.

U.S. Pat. No. 6,354,836 discloses methods of manufacturing dentalrestorations using CAD/CAM methods. For this, unsintered or pre-sinteredblocks of ceramic material and in particular aluminium oxide andzirconium oxide are used which result in high-strength dentalrestorations after milling to an enlarged shape followed by densesintering. However, it is considered to be essential that thetemperature differences in the sintering furnace used are smaller than10° C. in order to ensure that variations in the finally achieveddimensions of the restorations are small.

With the known pre-sintered blanks, the shrinkage occurring during thedense sintering and thus the enlargement factor to be applied depends toa great extent on the pre-sintering temperature applied. Even smallvariations, such as can occur as a result of an inhomogeneoustemperature distribution in the sintering furnace, result in differentshrinkages during the dense sintering. However, these shrinkages do notallow the desired small tolerances in the dimensions of the produceddental restoration.

The object of the invention is therefore to provide pre-sintered blankswhich avoid these disadvantages and are therefore less susceptible tovariations in the sintering temperature applied for their preparation.Likewise, these blanks should be able to be shaped easily by means ofcustomary grinding and milling processes to form dental restorationswith the desired geometry, without liquid needing to be supplied duringthese processes. Furthermore, these blanks should be able to beprocessed by dense sintering to form high-strength and optically veryattractive dental restorations.

This object is achieved by the pre-sintered blank according to claims 1to 9. Another subject of the invention is the process for thepreparation of the blank according to claims 10 and 11, the process forthe preparation of dental restorations according to claims 12 to 15 aswell as the use of the blank according to claim 16.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, details and features emerge from the followingdescription of an exemplary embodiment in conjunction with the drawings,in which:

FIG. 1 illustrates the enlargement factor plotted against thetemperature applied.

FIG. 2 illustrates the phases usually passed through during heattreatment of a glass powder green compact.

FIG. 3 illustrates a blank which has at least two areas which differ bytheir coloration or translucence.

FIG. 4 illustrates a blank with a holder attached to it.

The pre-sintered blank according to the invention for dental purposes ischaracterized in that it

is based on lithium disilicate glass ceramic and

has a relative density of from 60 to 90%, in particular 62 to 88% andpreferably 65 to 87%, relative to the true density of the glass ceramic.

The relative density is the ratio of the density of the pre-sinteredblank to the true density of the glass ceramic.

The density of the pre-sintered blank is determined by weighing it andascertaining its volume geometrically. The density is then calculatedaccording to the known formuladensity=mass/volume.

The true density of the glass ceramic is determined by grinding thepre-sintered blank to a powder with an average particle size of from 10to 30 μm, in particular of 20 μm, relative to the number of particlesand ascertaining the density of the powder by means of a pycnometer. Thedetermination of the particle size was carried out by means of laserdiffraction in accordance with ISO 13320 (2009) with the CILAS® ParticleSize Analyzer 1064 from Quantachrome GmbH & Co. KG.

It has surprisingly been found out that not only can the blank accordingto the invention be machined dry in a simple way, but it can also beprepared at significantly different pre-sintering temperatures, withoutthis resulting in a substantial change in the shrinkage which occursduring a subsequent dense sintering. The enlargement factor taking intoconsideration the shrinkage that occurs can thus be determined veryprecisely. These advantageous properties are clearly to be attributed tothe particular behaviour of lithium disilicate glass ceramic which waspre-sintered to the relative densities given above.

It is further preferred that the blank consists substantially of lithiumdisilicate glass ceramic. Particularly preferably, the blank consists oflithium disilicate glass ceramic.

The glass ceramic includes lithium disilicate as main crystal phase in apreferred embodiment. The term “main crystal phase” denotes the crystalphase which has the highest proportion by volume compared with othercrystal phases. In particular the glass ceramic contains more than 10vol.-%, preferably more than 20 vol.-% and particularly preferably morethan 30 vol.-% lithium disilicate crystals, relative to the total glassceramic.

The lithium disilicate glass ceramic contains SiO₂ and Li₂O, preferablyin a molar ratio in the range of from 1.75 to 3.0, in particular 1.8 to2.6.

In a further preferred embodiment, the lithium disilicate glass ceramiccontains at least one of the following components:

Component wt. % SiO₂ 50.0 to 80.0 Li₂O 6.0 to 20.0 Me(I)₂O 0 to 10.0, inparticular 0.1 to 10.0 Me(II)O 0 to 12.0, in particular 0.1 to 12.0Me(III)₂O₃ 0 to 8.0, in particular 0.1 to 8.0 Me(IV)O₂ 0 to 8.0, inparticular 0.1 to 8.0 Me(V)₂O₅ 0 to 8.0, in particular 0.1 to 8.0Me(VI)O₃ 0 to 8.0, in particular 0.1 to 8.0 nucleating agent 0 to 8.0,in particular 0.1 to 8.0

-   -   wherein    -   Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixtures        thereof,    -   Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures        thereof,    -   Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃ and        mixtures thereof,    -   Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂ and mixtures        thereof,    -   Me (V)₂O₅ is selected from Ta₂O₅, Nb₂O₅, V₂O₅ and mixtures        thereof,    -   Me(VI)O₃ is selected from WO₃, MoO₃ and mixtures thereof, and    -   nucleating agent is selected from P₂O₅, metals and mixtures        thereof.

Na₂O and K₂O are preferred as oxides of monovalent elements Me(I)₂O.

CaO, MgO, SrO and ZnO are preferred as oxides of divalent elementsMe(II)O.

Al₂O₃, La₂O₃ and Y₂O₃ are preferred as oxides of trivalent elementsMe(III)₂O₃.

ZrO₂, TiO₂ and GeO₂ are preferred as oxides of tetravalent elementsMe(IV)O₂.

Ta₂O₅ and Nb₂O₅ are preferred as oxides of pentavalent elementsMe(V)₂O₅.

WO₃ and MoO₃ are preferred as oxides of hexavalent elements Me(VI)O₃.

P₂O₅ is preferred as nucleating agent.

The lithium disilicate glass ceramic preferably contains colorantsand/or fluorescent agents.

Examples of colorants and fluorescent agents are inorganic pigmentsand/or oxides of d- and f-elements, such as the oxides of Ti, V, Sc, Mn,Fe, Co, Ta, W, Ce, Pr, Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids,e.g. of Ag, Au and Pd, can also be used as colorants and in addition canalso act as nucleating agents. These metal colloids can be formed e.g.by reduction of corresponding oxides, chlorides or nitrates during themelting and crystallization processes. For example, doped spinels,zircon silicate, stannates, doped corundum and/or doped ZrO₂ are used asinorganic pigments.

The blank according to the invention preferably has at least two areas,in particular layers, which differ in terms of their coloration ortranslucence. FIG. 3 shows a blank of at least two areas that differ interms of coloration or translucence. The blank preferably has at least 3and up to 10, particularly preferably at least 3 and up to 8, and evenmore preferably at least 4 and up to 6 areas, in particular layers,differing in coloration or translucence. The imitation of natural toothmaterial is very successful precisely because of the presence of severaldifferently coloured areas, in particular layers. It is also possiblethat at least one of the areas or of the layers has a colour gradient toensure a continuous colour transition.

In a further preferred embodiment, the blank according to the inventionhas a holder for securing it in a processing device. In anotherpreferred embodiment, the blank according to the invention has aninterface for connection to a dental implant. FIG. 4 shows a blankattached to a holder. The blank also includes an interface forconnection to a dental implant.

The holder allows the blank to be secured in a processing device, suchas in particular a milling or grinding device. The holder is usually inthe form of a pin and preferably consists of metal or plastic.

The interface ensures a connection between an implant and the dentalrestoration fitted thereon, such as in particular an abutment crown,which has been obtained by machining and dense sintering of the blank.This connection is preferably rotationally fixed. The interface ispresent in particular in the form of a recess, such as a bore. Thespecific geometry of the interface is usually chosen depending on theimplant system used in each case.

The invention also relates to a process for the preparation of the blankaccording to the invention, in which

-   (a) lithium silicate glass in powder or granulate form is pressed to    form a glass blank,-   (b) the glass blank is heat-treated in order to prepare a    pre-sintered blank based on lithium disilicate glass ceramic,    wherein the temperature of the heat treatment    -   (i) is at least 500° C., in particular at least 540° C. and        preferably at least 580° C., and    -   (ii) lies in a range which extends over at least 30° C., in        particular at least 50° C. and preferably at least 70° C. and in        which the relative density varies by less than 2.5%, in        particular less than 2.0% and preferably less than 1.5%.

In stage (a), lithium silicate glass in powder or granulate form ispressed to form a glass blank.

The lithium silicate glass employed is usually prepared by melting amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, for 2 to 10 h at temperatures of in particularfrom 1300 to 1600° C. To achieve a particularly high homogeneity, theobtained glass melt is poured into water in order to form a glassgranulate, and the obtained granulate is then melted again.

The granulate is then comminuted to the desired particle size and inparticular ground to powder with an average particle size of <100 μm,relative to the number of particles.

The granulate or powder is then, optionally together with added pressingauxiliaries or binders, usually placed in a compression mould andpressed to form a glass blank. The pressure applied lies in particularin the range of from 20 to 200 MPa. Uniaxial presses are preferably usedfor the pressing. The pressing can in particular also be isostaticpressing, preferably cold isostatic pressing.

Through the use of glass powders or glass granulates with differentcoloration or translucence, glass blanks can be produced which havedifferently coloured or differently translucent areas and in particularlayers. For example, differently coloured powders or granulates can bearranged on top of one another in a compression mould, with the resultthat a multi-coloured glass blank is produced. The multiple colours makeit possible to a great extent to give the finally prepared dentalrestorations the appearance of natural tooth material.

In stage (b), the obtained uni- or multi-coloured glass blank issubjected to a heat treatment in order to bring about the controlledcrystallization of lithium disilicate and thus the formation of alithium disilicate glass ceramic as well as the pre-sintering. The heattreatment takes place in particular at a temperature of from 500° C. to900° C., preferably from 540 to 900° C. and particularly preferably from580 to 900° C. The heat treatment is carried out in particular for aperiod of from 2 to 120 min, preferably 5 to 60 min and particularlypreferably 10 to 30 min.

The temperature range (b)(ii) describes a range in which, despite achange in temperature, the relative density hardly changes. This rangeis therefore also called “plateau” in the following. The variation inthe relative density possible in this range is calculated in % from themaximum and minimum value of the relative density in the range by(maximum value−minimum value)/maximum value×100

It has surprisingly been shown that during its production andpre-sintering in particular temperature ranges lithium disilicate glassceramics display essentially no change in the relative density and thusin the linear shrinkage and the enlargement factor during the densesintering. These ranges are recognizable as “plateaus” in the graphicrepresentation of relative density, linear shrinkage or enlargementfactor against the temperature. Accordingly, properties of the blankthat are important for the accuracy of fit of the later dentalrestoration are essentially not dependent on the temperature in thisrange. The result of this is the important practical advantage that theblank tends to be unsusceptible e.g. to temperature fluctuations ortemperature gradients in the sintering furnace, as long as thetemperature is in the “plateau” range.

According to the invention, therefore, pre-sintered blanks which areobtainable by the process according to the invention are particularlypreferred.

Particularly preferred are blanks according to the invention which havea relative density which results when

-   (a) powder of a corresponding starting glass with an average    particle size of <100 μm, relative to the number of particles, is    uniaxially or isostatically pressed at a pressure of from 20 to 200    MPa, preferably 40 to 120 MPa and particularly preferably 50 to 100    MPa and-   (b) the obtained glass powder green compact is heat-treated for 2 to    120 min, preferably 5 to 60 min and particularly preferably 10 to 30    min at a temperature which    -   (i) is at least 500° C., in particular at least 540° C. and        preferably at least 580° C., and    -   (ii) lies in a range which extends over at least 30° C., in        particular at least 50° C. and preferably at least 70° C. and in        which the relative density varies by less than 2.5%, in        particular less than 2.0% and preferably less than 1.5%.

FIG. 2 illustrates the phases usually passed through during heattreatment of a glass powder green compact by plotting the enlargementfactor against the temperature for a green compact with a compositionaccording to Example 2. In Phase I, up to about 500° C., the heating andthe removal of any binder present take place. In Phase II, from about500 to 600° C., sintering and crystallization take place, and in PhaseIII, the plateau, from about 600 to about 850° C., there is apre-sintered blank according to the invention based on lithiumdisilicate glass ceramic. Then, in Phase IV, from about 850 to about950° C., the dense sintering of the blank takes place.

The pre-sintered blank according to the invention is preferably presentin the form of blocks, disks or cylinders. In these forms, a furtherprocessing to form the desired dental restorations is particularly easy.

The pre-sintered blank is further processed in particular to form dentalrestorations. The invention therefore also relates to a process for thepreparation of dental restorations, in which

(i) the pre-sintered blank according to the invention based on lithiumdisilicate glass ceramic is shaped by machining to form a precursor ofthe dental restoration,

(ii) the precursor is substantially dense sintered in order to producethe dental restoration, and

(iii) optionally the surface of the dental restoration is provided witha finish.

In stage (i), the machining is usually carried out by material removalprocesses and in particular by milling and/or grinding. It is preferredthat the machining is carried out with computer-controlled millingand/or grinding devices. Particularly preferably, the machining iscarried out as part of a CAD/CAM process.

The blank according to the invention can be machined very easily inparticular because it is open-pored and has low strength. It isparticularly advantageous that it is not necessary to use liquids duringthe grinding or milling. In contrast to this, so-called wet-grindingprocesses are often necessary for conventional blanks.

The machining is usually carried out in such a way that the obtainedprecursor represents an enlarged form of the desired dental restoration.The shrinkage occurring during the subsequent dense sintering is therebytaken into consideration. The blank according to the invention has theparticular advantage that the enlargement factor to be applied to it canbe determined very precisely. The enlargement factor is the factor bywhich the precursor has to be ground or milled enlarged out of thepre-sintered blank in order that after the dense sintering the obtaineddental restoration has the desired dimensions.

The enlargement factor F_(v), the relative density ρ_(r) and theremaining linear shrinkage S can be converted into each other asfollows:S=1−ρ_(r) ^(1/3)F _(v)=1/(1−S)

In a preferred embodiment, the blank produced according to theabove-described process according to the invention is used aspre-sintered blank.

In stage (ii) the obtained precursor is substantially dense-sintered inorder to produce the dental restoration with the desired geometry.

For the dense sintering, the precursor is preferably heat-treated at atemperature of from 700 to 1000° C. The heat treatment usually takesplace for a period of from 2 to 30 min.

After the dense sintering, there is a dental restoration based onlithium disilicate glass ceramic in which lithium disilicate preferablyforms the main crystal phase. This lithium disilicate glass ceramic hasexcellent optical and mechanical properties as well as a high chemicalstability. Dental restorations which meet high demands can thus beprepared with the process according to the invention.

The dental restorations are preferably selected from crowns, abutments,abutment crowns, inlays, onlays, veneers, shells and bridges as well asoverstructures for multi-part restoration frames which can consist e.g.of oxide ceramic, metals or dental alloys.

It can be advantageous for the dense sintering that the precursor of thedental restoration is supported in order to avoid a distortion. It ispreferred that the support consists of the same material as theprecursor and hence shows the same shrinkage upon sintering. The supportcan be in form of for example a supporting structure or supporting mouldwhich in terms of their geometry are adapted to the precursor.

In the optional stage (iii), the surface of the dental restoration canalso be provided with a finish. It is possible in particular to alsocarry out a glazing firing at a temperature of from 650 to 900° C. or topolish the restoration.

Because of the described properties of the pre-sintered blank accordingto the invention, it is suitable in particular for producing dentalrestorations. The invention therefore also relates to the use of theblank to prepare dental restorations and in particular crowns,abutments, abutment crowns, inlays, onlays, veneers, shells and bridgesas well as overstructures.

The average particle sizes given, relative to the number of particles,were determined at room temperature by laser diffraction with the CILAS®Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KG inaccordance with ISO 13320 (2009).

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 4

A total of 4 glass ceramics with lithium disilicate as main crystalphase with the composition given in Table I were prepared by meltingcorresponding starting glasses and then, by heat treatment, presinteringglass powder blanks produced from them, and crystallizing them in acontrolled manner.

For this, the starting glasses on a scale of 100 to 200 g were firstmelted from customary raw materials at 1400 to 1500° C., wherein themelting could be carried out very easily without formation of bubbles orstreaks. By pouring the starting glasses into water, glass frits wereprepared which were then melted a second time at 1450 to 1550° C. for 1to 3 h for homogenization.

The obtained glass melts were then cooled to 1400° C. and converted tofine-particle granulates by pouring into water. The granulates weredried and ground to powder with an average particle size of <100 μm,relative to the number of particles. These powders were moistened withsome water and pressed to form powder green compacts at a pressingpressure of from 20 to 200 MPa.

The powder green compacts were then heat-treated for 2 to 120 min at atemperature which lies in the range given as plateau in Table I for therespective composition. After this heat treatment, blanks according tothe invention were present which were pre-sintered and based on lithiumdisilicate glass ceramic.

TABLE I Example 1 2 3 4 Component wt. % wt. % wt. % wt. % SiO₂ 67.5 75.278.4 69.4 Li₂O 14.9 15.6 16.3 19.7 P₂O₅ 4.3 — 3.3 3.4 K₂O 4.2 — — — MgO0.7 — — — SrO — 4.1 — — ZnO 4.8 — — — Al₂O₃ — 3.6 — 3.5 La₂O₃ 1.0 — — —Er₂O₃ — 0.25 — — CeO₂ 2.0 1.0 — — SnO₂ — — 2.0 — Nb₂O₅ — — — — V₂O₅ 0.1— — — MoO₃ — — — 4.0 Tb₄O₇ 0.5 — — — Main crystal phase LS2 LS2 LS2 LS2Plateau (° C.) 600- 590- 600- 600- 800 860 900 900 LS2 lithiumdisilicate

Example 5 Examination of Sintering Behaviour of the CompositionAccording to Example 1

A glass with the composition according to Example 1 was melted andground to a glass powder with an average particle size of less than 50μm, relative to the number of particles. This glass powder was pressedto form cylinders. The sintering behaviour of these cylindrical blankswas examined by heat-treating them at different temperatures in afurnace of the Programat® P500 type from Ivoclar Vivadent AG. In eachcase a heating rate of 20° C./min and a holding time of 2 min at therespective temperature were used. After that the blanks were cooled toroom temperature and the relative density of the blanks was thendetermined in each case in relation to the true density of the glassceramic. The remaining linear shrinkage as well as the enlargementfactor were calculated from the relative density.

The results for temperatures in the range of from 25 to 900° C. areshown in the following Table II. A pre-sintered lithium disilicate glassceramic blank according to the invention with a relative density of from69 to 70% was present at between 600 and 800° C.

TABLE II Temperature [° C.] 25 450 500 550 600 650 700 750 800 850 900Diameter [mm] 16.10 16.09 16.07 15.47 14.55 14.59 14.61 14.56 14.4813.39 13.12 Height [mm] 15.16 15.14 15.05 14.91 13.81 13.80 13.83 13.8813.92 12.65 12.03 Volume [cm3] 3.09 3.08 3.05 2.80 2.30 2.31 2.32 2.312.29 1.78 1.63 Mass [g] 4.00 3.98 3.99 4.05 3.98 3.98 3.97 3.99 4.014.03 3.98 Density [g/cm3] 1.30 1.29 1.31 1.45 1.74 1.73 1.71 1.73 1.752.26 2.45 Relative density [%] 52 52 52 58 69 69 69 69 70 91 98 Linearshrinkage [%] 19.6 19.5 19.2 17.3 11.4 11.5 11.6 11.6 11.5 3.5 0.0Enlargement factor 1.24 1.24 1.24 1.21 1.13 1.13 1.13 1.13 1.13 1.041.00

In FIG. 1, the calculated enlargement factor is plotted against thetemperature applied. It can be seen from this that the enlargementfactor surprisingly remains substantially constant in the range of from600 to 800° C. and the curve forms a plateau. Thus, when a heattreatment is applied in this range, a blank according to the inventioncan be produced for which a very precise specification of theenlargement factor to be chosen is possible.

The same process for determining this range (“plateau”) was also usedfor the other compositions given in Table I.

Example 6 Examination of Sintering Behaviour of the CompositionAccording to Example 2

The sintering behaviour of the composition according to Example 2 wasexamined analogously to Example 5. The values obtained for the relativedensity, the remaining linear shrinkage and the enlargement factor arelisted in Table III.

TABLE III Temperature [° C.] 30 450 500 550 600 650 700 750 800 850 900950 Linear shrinkage [%] 14.05 14.10 14.30 12.80 11.45 11.45 11.55 11.4511.40 11.45 9.95 0.00 Enlargement factor 1.163 1.164 1.167 1.147 1.1291.129 1.131 1.129 1.129 1.129 1.110 1.000 Relative density [%] 63 63 6366 69 69 69 69 70 70 73 100

The enlargement factor was plotted against the temperature and the curveobtained is shown in FIG. 2. It can be seen from this that the plateaufor the examined lithium disilicate glass ceramic is in a range of about600 to about 850° C. A pre-sintered lithium disilicate glass ceramicblank according to the invention with a relative density of from 69 to70% is present in this range.

The invention claimed is:
 1. Pre-sintered blank for dental purposesbased on lithium disilicate glass ceramic, wherein the blank has arelative density of from 60 to 90%, relative to the true density of theglass ceramic, and wherein the blank has a relative density whichresults when (a) powder of a corresponding starting glass with anaverage particle size of <100 μm, relative to the number of particles,is uniaxially or isostatically pressed at a pressure of from 20 MPa to200 MPa and (b) the obtained glass powder green compact is heat-treatedfor 2 to 120 min at a temperature (i) which temperature is at least 500°C., and (ii) which temperature lies in a temperature range which extendsover at least 30° C. and in which temperature range the relative densityvaries by less than 2.5%.
 2. Pre-sintered blank according to claim 1,which consists essentially of lithium disilicate glass ceramic. 3.Pre-sintered blank according to claim 1, wherein the glass ceramicincludes lithium disilicate as main crystal phase.
 4. Pre-sintered blankaccording to claim 3, wherein the glass ceramic contains more than 10vol.-% lithium disilicate crystals.
 5. Pre-sintered blank according toclaim 3, wherein the glass ceramic contains more than 20 vol.-% lithiumdisilicate crystals.
 6. Pre-sintered blank according to claim 1, whereinthe lithium disilicate glass ceramic contains at least one of thefollowing components: Component wt. % SiO₂ 50.0 to 80.0 Li₂O  6.0 to20.0 Me(I)₂O   0 to 10.0 Me(II)O   0 to 12.0 Me(III)₂O₃   0 to 8.0Me(IV)O₂   0 to 8.0 Me(V)₂O₅   0 to 8.0 Me(VI)O₃   0 to 8.0 nucleatingagent    0 to 8.0,

wherein Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixturesthereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixturesthereof, Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃and mixtures thereof, Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂and mixtures thereof, Me(V)₂O₅ is selected from Ta₂O₅, Nb₂O₅ andmixtures thereof, Me(VI)O₃ is selected from WO₃, MoO₃ and mixturesthereof, and nucleating agent is selected from P₂O₅, metals and mixturesthereof.
 7. Pre-sintered blank according to claim 1, which has at leasttwo areas which differ by their coloration or translucence. 8.Pre-sintered blank according to claim 7, which has at least two layerswhich differ by their coloration or translucence.
 9. Pre-sintered blankaccording to claim 1, which has a holder for securing the blank to aprocessing device.
 10. Pre-sintered blank according to claim 1, whichhas an interface for connection to a dental implant.
 11. Pre-sinteredblank according to claim 10, wherein the interface comprises a recessfor connection to the dental implant.
 12. Pre-sintered blank accordingto claim 1, wherein the blank has a relative density of from 62 to 88%,relative to the true density of the glass ceramic.
 13. Pre-sinteredblank according to claim 1, wherein the blank has a relative density offrom 65 to 87%, relative to the true density of the glass ceramic. 14.Pre-sintered blank according to claim 1, wherein the lithium disilicateglass ceramic contains at least one of the following components:Component wt. % SiO₂ 50.0 to 80.0 Li₂O  6.0 to 20.0 Me(I)₂O  0.1 to 10.0Me(II)O  0.1 to 12.0 Me(III)₂O₃ 0.1 to 8.0 Me(IV)O₂ 0.1 to 8.0 Me(V)₂O₅0.1 to 8.0 Me(VI)O₃ 0.1 to 8.0 nucleating agent  0.1 to 8.0,

wherein Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixturesthereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixturesthereof, Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃and mixtures thereof, Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂and mixtures thereof, Me(V)₂O₅ is selected from Ta₂O₅, Nb₂O₅ andmixtures thereof, Me(VI)O₃ is selected from WO₃, MoO₃ and mixturesthereof, and nucleating agent is selected from P₂O₅, metals and mixturesthereof.
 15. Pre-sintered blank according to claim 1, which has arelative density which results when (a) powder of a correspondingstarting glass with an average particle size of <100 μm, relative to thenumber of particles, is uniaxially or isostatically pressed at apressure of from 40 to 120 MPa and (b) the obtained glass powder greencompact is heat-treated for 5 to 60 min at a temperature (i) whichtemperature is at least 540° C., and (ii) which temperature lies in atemperature range which extends over at least 50° C. and in whichtemperature range the relative density varies by less than 2.0%. 16.Pre-sintered blank according to claim 1, which has a relative densitywhich results when (a) powder of a corresponding starting glass with anaverage particle size of <100 μm, relative to the number of particles,is uniaxially or isostatically pressed at a pressure of from 50 to 100MPa and (b) the obtained glass powder green compact is heat-treated for10 to 30 min at a temperature (i) which temperature is at least 580° C.,and (ii) which temperature lies in a temperature range which extendsover at least 70° C. and in which temperature range the relative densityvaries by less than 1.5%.
 17. Process for the preparation of dentalrestorations, in which (i) the pre-sintered blank based on lithium disilicate glass ceramic according to claim 1 is shaped by machining toform a precursor of the dental restoration, (ii) the precursor issubstantially dense-sintered in order to produce the dental restoration,and (iii) optionally the surface of the dental restoration is providedwith a finish.
 18. Process according to claim 17, in which the machiningis carried out with computer-controlled milling and/or grinding devices.19. Process according to claim 17, in which (a) lithium silicate glassin powder or granulate form is pressed to form a glass blank, (b) theglass blank is heat-treated in order to prepare a pre-sintered blankbased on lithium disilicate glass ceramic, (i) wherein the temperatureof the heat treatment is at least 500° C., and (ii) wherein thetemperature of the heat treatment lies in a temperature range whichextends over at least 30° C. and in which temperature range the relativedensity varies by less than 2.5% and is carried out in order to obtainthe pre-sintered blank based on lithium disilicate glass ceramic. 20.Process according to claim 17, in which the dental restorations areselected from crowns, abutments, abutment crowns, inlays, onlays,veneers, shells, bridges and overstructures.
 21. A method of using theblank according to claim 1 to prepare dental restorations comprisingcrowns, abutments, abutment crowns, inlays, onlays, veneers, shells,bridges and overstructures.